1. Field of the Invention
This invention relates to an apparatus and process for quenching heated steel parts.
2. Description of Prior Developments
The properties of steel parts can sometimes be improved by rapidly cooling such parts from a relatively high temperature, e.g. above about 1500.degree. F., down to a relatively low temperature, e.g. approximately 200.degree. F. By carrying out the cooling process in a relatively short time period, e.g. about one or two seconds, the steel is metallurgically transformed into a relatively hard condition, useful for many product applications. This rapid cooling process is usually termed quenching.
In many cases the quenching process is performed by plunging the steel part into a bath of relatively cool liquid. A bath temperature of around 80.degree. F. is common. The quenching liquid can be any liquid that effectively removes heat from the steel part such as water, a sodium chloride brine solution, or oil.
The quenching (cooling) process should be performed at a relatively fast rate. However, if the cooling is too rapid or uneven the part may undergo thermal distortion due to the fact that some areas of the part are momentarily in contraction while other areas are not. This could lead to cracking of the part. The problem is somewhat complicated by the fact that the cooling rate is affected by the turbulence or non-turbulence of the liquid at the part-liquid interface.
It is known that the heat exchange rate can be appreciably increased by causing the liquid to be in a turbulent state. The turbulent liquid exerts a scrubbing action on the part, which disturbs and replaces the film on the surface of the part, such that the disturbed liquid is enabled to remove heat from the part and provide new cool liquid on the part surface.
In order to promote heat transfer between the liquid and the part as it is being plunged into the coolant bath, it has been proposed to move the coolant upwardly around and across the part, preferably in a turbulent flow. In one known arrangement the heated part is dropped into a vertical flow tube containing a quenching liquid (oil). A pump is provided for moving the liquid upwardly through the tube. The downwardly-moving part makes thermal contact with the upwardly-moving liquid, such that some turbulence is created at the interface between the liquid and the contacting surface of the part.
U.S. Pat. No. 3,684,263 to Genrich discloses a quench tank of the above-mentioned type wherein an upward flow of cooling liquid is generated by a motor-operated pump. In experimentation with quenching systems wherein the liquid is pumped upwardly through a vertical flow tube, it was discovered that the linear flow rate is often not uniform across the tube cross section. In one system studied, the linear flow rate varied from a low value of about 1.6 feet per second to a high of about 3.0 fee per second in a given cross section. The variation is at least partly attributable to the fact that the liquid enters the tube through a side opening in the tube wall and has a lateral motion component that is never fully eliminated.
The non-uniform liquid flow through the tube produces variations in the turbulence at different sections of the downwardly-moving part. The part surface areas making contact with the fastest (most turbulent) liquid tend to be cooled at a more rapid rate than the other surface areas. There tends to be a degree of thermal distortion in the part, and also an undesired variation in the hardness of the part, from one area of the part to another. There also tends to be some hardness variations from one part to another part.